Insulating coatings for electrical insulators and spray material for such coatings



y 0, 1955 R. H. ZACHARIASON 2,708,168

INSULATING COATINGS FOR ELECTRICAL INSULATORS 7 AND SPRAY MATERIAL FOR SUCH COATINGS I Filed Feb. 28, 1951 INVENTOR RIcHARnI'LZAnHAmAsuN myw ATTORNEY United States Patent INSULATING COATINGS FOR ELECTRICAL IN- SULATORS AND SPRAY MATERIAL FOR SUCH COATINGS Richard Howard Zachariason, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application February 28, 1951, Serial No. 213,202

6 Claims. (Cl. 106-84) The present invention relates to spray materials for improving the usefulness of electrical insulators, and more particularly to a novel spray material for providing coatings of new and desirable characteristics on insulating spacers employed in electron tubes.

One type of insulating spacer used in a relatively large variety of electron tubes comprises a fiat disc or plate made of insulating material such as mica.. The disc or plate has a plurality of apertures for receiving end portions of electrodes.

It has heretofore been found desirable to coat the surfaces of the spacer plate for providing a rough surface thereon to prevent electrical leakage across the plate between the end portions of two or more electrodes. Such leakage results from metal depositions on the spacer having their source in evaporated cathode and getter material evolved during normal exhaust procedure in fabrication of electron tubes. A coating material heretofore commercially used is magnesium oxide.

This material has serious limitations. One of these limitations is its poor adhesion to surfaces of the spacer. This poor adhesion has proven particularly troublesome in handling tube mounts having spacers so coated. Such handling, even though accompanied by care, has resulted in appreciable loss of the coating. A further limitation is the relatively low waterresistance of the material. This low water resistance results in further loss of coating when a mount including a coated spacer is washed in water. Such washing is a necessary step in tube manufacture to remove foreign material that may adhere to mount elements. A further problem resides in the abrasiveness of prior coatings that causes excessive wear on processing tools.

A particularly troublesome feature of the presently used coating referred to is its high electrical resistance. This feature presents a serious problem in relatively high voltage tubes such as those used in television receiving circuits. In view of this high electrical resistance of prior art coatings, they become charged in response to electron bombardment necessary during normal use of the tube. Such electron bombardment on insulating spacers usually releases fewer secondary electrons than the bombarding primary electrons, with the result that a negative charge is built up on the spacer surfaces. Due to the high electrical resistance of the coating on the spacer, this charge is not dissipated in small amounts, but builds up to an appreciable magnitude. When the charge is sufficiently high, insulation breakdown between the charged surface and adjacent positive or less negative elements of the tube occurs. This breakdown causes erratic tube operation, resulting in noise in an audio amplifier associated with the tube or in streaking in a video picture.

The foregoing difficulties are overcome in accordance with the invention by the provisions of a coating material comprising a mixture of aluminum oxide, a binder that is water resistant when dried, such as an inorganic silicate of an alkaline material such as sodium or potassium, and water as the solvent, in predetermined relative amounts 2,708,168 Patented May 10, 1955 within certain tolerable ranges. The range of content of the aluminum oxide may be further modified in accordance with the particle size of this material. After the coating material is applied to a spacer plate and water driven off and the silicate dehydrated, the resultant coating is substantially insoluble in water, andis characterized by reduced electrical resistance and improved adherence on the spacer plate.

An important object of the invention is to provide an improved coating material for application as a coating to insulating elements.

Another object is to provide an insulating coating that is substantially insoluble in water.

A further object is to provide a coating on electrical insulators, having reduced electrical resistance.

Another object is to provide an insulating coating characterized by good adherence on a base.

A further object is to provide an insulating coating having reduced abrasiveness for preserving processing tools against wear.

Another object is to provide a coating on insulating spacer plates of electron tubes having improved adherence, reduced abrasiveness, freedom from solution in water to permit washing said plates, and reduced electrical resistance for preventing accumulation thereon of charges of appreciable magnitude.

Further objects and advantages will appear from the following detailed description of the invention taken in connection with the appended drawing in which the sole figure is a transverse view of an electron tube showing the top of the mount thereof including an insulating spacer plate having a coating in accordance with the invention.

Referring to the drawing in more detail, there is shown an electron tube having an envelope 10, and electrodes comprising a cathode, anode and grid having end portions 11, 12 and 13 respectively. For spacing the electrodes with respect to each other and with respect to the inner walls of envelope 10, a spacer plate 14 of insulating material such as mica is provided. The spacer plate has an insulating coating 15.

According to the invention, the insulating coating 15 comprises aluminum oxide and a silicate as binder. The aluminum oxide has a particle size of from 0.02 to 4.0 microns in diameter for a purpose to be described. The silicate is preferably of low alkalinity for improved stability. One silicate used is sodium silicate comprising one part of NazO to 3.22 parts of SiOz. This silicate has a specific gravity of 41 Baum and a viscosity of 1.8 poises. Another silicate I have used is potassium silicate, in the relative amounts of one part K20 and 3.3 parts SiOz. Other silicates have also been used to advantage. However, potassium silicate is preferred because it is easier to degas than other silicate compounds. The silicate confers the properties of reduced electrical resistance of the coating, insolubility of the coating in water, and improved adherence of the coating on a base.

The coating 15 is applied to the spacer plate 14 by spraying the coating material of the invention on the plate. The coatingsmaterial according-to. the invention comprises Wymoxide A1203 a ilicate such as K2O.3.3Si6z and water i n pr fietgg mgred ggliygam o finti The amount of aluminum oxide in th e c oating material is dependent on the particle size of the aluminum oxide powder used. An increase in the particle size requires an increase in the relative amount of aluminum oxide employed. I have found a particle size of the aluminum oxide of from 0.02 micron to 4.0 microns in diameter to be best suited for the purposes of the invention. This is known as a fluify grade of aluminum oxide.

The coating material according to one example of the invention, includes the following relative amounts of aluminum oxide, silicate and water within the ranges specified.

Grams A1203 (particle size of 0.02 micron) 114-126 Silicate 57 /286 /& Water 570-630 When the particle size of the aluminum oxide powder is 4.0 microns in diameter, the amount of the aluminum oxide in the material should be from 1140 to 1260 grams, with the amounts of silicate and water above indicated remaining unchanged.

My imp'iov ed coating material may also use aluminum oxide having a particle size intermediate 0.02 and 4.0 microns. When aluminum oxide of such intermediate particle size is used, the relative amount thereof in my coating material should be somewhere between 114 and 1260 grams.

The actual amount used is related to the extremes of the quantity range referred to in same manner as the particle size of the aluminum oxide is related to the extremes of the size range mentioned above. For example, if the particle size of the aluminum oxide is of an intermediate value such as 2.01 microns in diameter, then the quantity thereof used in my coating material should be about 687 grams, or an intermediate value of the quantity range referred to, the acceptable sub-range being from 653 to 721 grams. I have found that for any particle size between 0.02 and 4.0 microns the quantity of aluminum oxide in my novel material may vary up or down from a median value within a range of from 120 to 1200 grams by an amount equal to 5 per cent by weight of said median value. This is illustrated in the example referred to wherein the median value of 687 grams between the extremes of 120 and 1200 of the overall size range is permitted a variation according to the invention of 5 per cent up to 721 grams and a variation down to 653 grams. The extremes of 114 and 1260 grams referred to above represent extremes of both the overall size range as well as extremes of the upper and lower terminal sub-ranges. Thus, the lower extreme of 114 grams is the lower extreme of the overall size range as well as the lower extreme of the lower sub-size range of from 114 to 126 grams.

These variations in the relative amounts of aluminum oxide permissible in my coating material, do not involve variations in the amounts of the silicate and water used, other than within the ranges specified. The relative amounts of silicate and water in the coating material of the invention are readily determined from the formula referred to. silicate, aluminum oxide and water, in relative amounts according to the invention, these amounts may be considered to have approximate or rough values. When so considered, the slight variation in the amounts of the ingredients as measured out, will not be objectionable since the amounts will lie within the ranges specified. Thus the silicate contenttaccording to. the invention may be stated area about 72 parts by weight. This would permit some variation in the silicate amount but 1M0 less manila or tg more han.86 /2.parts by weight, as aforementioned. The amount of aluminum oxide within the overall size range of from UJMO .1260 parts by weight, may for convenience in measuring out, be stated to consist of from about 120E ,abouLlZQQlQntsbyyveight. In like manner, the amount of water required for a coating material according to the invention may be conveniently stated to be about 600 parts by weight which is intermediate the extremes of 570 and 630 parts specified for these ingredients.

Each of the constituents of my novel coating material possesses characteristics rendering it particularly advantageous for forming an improved insulating coating on electrical insulators. For example, the aluminum oxide is a very stable compound capable of withstanding relatively high temperatures. It has excellent insulating properties, is inert, and will not break down to release gas under electron bombardment occurring in electron tubes. Fur- Indeed, to facilitate a measuring out of the thermore, the critical size range to which the particles of the aluminum oxide are limited according to the invention, improves the suitability of the compound as a constituent of a coating material. For example, the relatively small particle size tolerated, contributes to reduced abrasiveness of the coating. This is important in connection with processing coated elements. The abrasive nature of the coated elements causes wear on processing tools, such as stitching machines by which insulating spacers for electron tubes are processed. The reduction in abrasiveness resulting from the coating of the invention is therefore accompanied by reduced wear on processing tools.

With respect to the silicates used, such as Na2O.3.22SiO2 and K2O.3.3SiO2, their low alkalinity renders them more stable than silicates or compounds of high alkalinity. Moreover, the silicate serves a novel function in insulating coatings. This function is that of a binder in the applied coating. In addition, it serves to effectively join the coating to a base, so that the coating is substantially free from separation from the base during normal handling. In addition, the silicate employed according to the invention is substantially insoluble in water after dehydration during formation of the coating. This permits the coated elements, such as insulating spacer plates in electron tubes, to be washed in water without harm to the coating. Furthermore, the silicates reduce the electrical resistance of the applied coating, so that electrical charges are prevented from building up excessively thereon. This is of particular importance in electron tubes used for television reception wherein certain of the electrodes therein are operated at a relatively high positive potential. Moreover, the silicate is not harmed during bakeout of electron tubes, at temperatures as high as 400 to 500 C.

Water is used as the solvent in the coating material of the invention in view of the solubility of the silicates therein. The silicates are not soluble in organic compounds but are precipitated thereby. The use of water furthermore is advantageous from an economical standpoint and also because it is readily driven off during formation of a coating.

The coating material according to the invention is applied by a spray gun preferably adjusted to produce a dry spray. A spray of this type will dry as soon as it impinges on the base to be coated, and is dry to the touch at all times during the spraying operation.

Where the coating is applied to an insulating spacer plate made of mica, an acceptable thickness is characterized by a tolerance that permits visual control. Thus, the coating should be applied thickly enough to provide a uniform white covering on the spacer plate.

It will be appreciated from the foregoing that a novel and advantageous coating material is provided as well as an improved coating formed by said material. The coating is particularly useful on electrical insulators such as insulating spacer plates employed in electron tubes and contributes appreciably to improved operation of such tubes.

What is claimed is:

1. A coating material for application as an insulating coating on electrical insulators, consisting of about 600 parts by weight of water as' solvent, about 72 parts by weight of a silicate of low alkalinity as binder selected from the group consisting of sodium silicate and potassium silicate, and aluminum oxide, the amount of said aluminum oxide being from about 120 to about 1200 parts by weight, the particle size of said aluminum oxide being within the range of from 0.02 to 4.0 microns diameter, the amount of said aluminum oxide being directly related to said particle size to provide from about 120 parts of said oxide at a particle size of 0.02 micron, to about 1200 parts of said oxide at a particle size of 4.0 microns, for providing a stable coating of reduced electrical resistance and reduced abrasiveness.

2. A sprayable coating material consisting of from 570 to 630 parts by weight of water, 57.5 to 86.5 parts by weight of a silicate of low alkalinity selected from the group consisting of sodium silicate and potassium silicate, and from about 120 to about 1200 parts by weight of aluminum oxide having a particle size of from 0.02 to 4.0 microns in diameter, said 120 parts having said particle size of 0.02 microns and said 1200 parts having said particle size of 4.0 microns, amounts of said aluminum oxide intermediate said 120 and 1200 parts having a particle size intermediate said 0.02 and 4.0 microns, said material having desirable properties for spraying on electrical insulators to provide a coating thereon of improved electrical characteristics.

3. An insulating spacer for an electron tube comprising a structure having thereon an insulating coating that is readily degasable, for preserving a desired vacuum in said tube, said coating consisting of from 57.5 to 86.5 parts by weight of potassium silicate of low alkalinity, and from about 120 to about 1200 parts by weight of aluminum oxide having a particle size increasing from 0.02 to 4.0

microns in diameter with increased amounts of said oxide.

4. A coating material adapted to be sprayed on insulating spacers of electron tubes to provide a coating thereon having reduced electrical resistance for dissipation of spot charges thereon of electrical energy, and having the property of being readily degasable to prevent contamination of a desired vacuum in such tubes, and having reduced abrasiveness for reducing wear on tools employed 'in processing the coated spacers, said material consisting of from 570 to 630 parts by weight of water, 57.5 to 86.5 parts by weight of potassium silicate having in chemical combination one part by weight of potassium oxide to 3.3 parts by weight of silicon dioxide, and from 114 to 1260 parts by weight of aluminum oxide having a particle size from 0.02 to 4.0 microns in diameter.

5. A coating material for use in coating electrical insulators to reduce incidents of appreciable build-up thereon of electrical charges; said material consisting of from 570 to 630 parts by weight of water; from 57.5 to 86.5 parts by weight of a silicate of low alkalinity selected from the group consisting of sodium silicate and potassium silicate; and aluminum oxide in an amount and of a particle size of from 114 parts by weight at 0.02 microns in diameter to 1260 parts by weight at 4.0 microns in diameter.

6. A coating material consisting of by weight from 570 to 630 parts of water, from 57.5 to 86.5 parts of a silicate of low alkalinity selected from the group consisting of sodium silicate and potassium silicate, and aluminum oxide in a range from 114 to 1260 parts and having a particle size in a range of from 0.02 to 4.0 microns in di ameter, the amount of said aluminum oxide in said first named range being increased with increases in the particle size in said second named range.

References Cited in the file of this patent UNITED STATES PATENTS 49,708 Brown Sept. 5, 1865 1,606,481 Rochow Nov. 9, 1926 1,713,580 Williams May 21, 1929 1,829,237 Ruben Oct. 27, 1931 1,924,311 Frey Aug. 29, 1933 2,065,389 Mohrle Dec. 22, 1936 2,171,230 Wamsley Aug. 29, 1939 2,281,834 Dietz et al. May 5, 1942 2,545,120 Swedlund Mar. 13, 1951 FOREIGN PATENTS 398,767 Great Britain Sept. 21, 1933 

1. A COATING MATERIAL FOR APPLICATION AS AN INSULATING COATING ELECTRICAL INSULATORS, CONSISTING OF ABOUT 600 PARTS BY WEIGHT OF WATER AS SOLVENT, ABOUT 72 PARTS BY WEIGHT OF A SILICATED OF LOWER ALKALINITY AS BINDER SELECTED FROM THE GROUP CONSISTING OF SODIUM SILICATE AND POTASSIUM SILICATE, AND ALUMINUM OXIDE, THE AMOUNT OF SAID ALMINUM OXIDE BEING FROM ABOUT 120 TO ABOUT 1200 PARTS BY WEIGHT, THE PARTICLE SIZE OF SAID ALUMINUM OXIDE BEING WITHIN THE RANGE OF FROM 0.02 TO 4.0 MICRONS DIAMETER, THE AMOUNT OF SAID ALUMINUM OXIDE BEING DIRECTLY RELATED TO SAID PARTICLE SIZE TO PROVIDE FROM ABOUT 120 PARTS OF SAID OXIDE AT A PARTICLE SIZE OF 0.02 MICRON, TO ABOUT 1200 PARTS OF SAID OXIDE AT A PARTICLE SIZE OF 4.0 MICRON, FOR PROVIDING A STABLE COATING OF REDUCED ELECTRICAL RESISTANCE AND REDUCED ABRASIVENESS. 